According to the Poly VVX D230 DECT IP Phone Administrator Guide 7.0 (https://support.polycom.com/content/dam/polycom-support/products/voice/dect-ip-phones/user/en/vvxd230-administrator-guide.pdf) you can enable call waiting on the handset by dialing *56, but if two calls come at the same time the phone returns a 486 busy.
Is it possible to increase the number of calls a handset can handle at the same time?